1. Field of the Invention
This invention relates to a process for producing an alumina-zirconia composite sintered material which is useful for structural materials, electronic materials, heat-resistant and heat-insulating materials, cutting tool materials, grinding materials, polishing materials, etc.
2. Description of the Prior Art
It is said that a sintered material (a so-called ceramic) of an oxide or non-oxide inorganic powder, when produced at lower temperatures so as to have a high density, has a high mechanical strength because the growth of constituent grains in the sintered material is suppressed. The lower sintering temperature leads to savings in energy and enables the use of various easier methods for obtaining the necessary temperature.
Hence, production processes for various inorganic powders have been developed. As to production of alumina-zirconia composite powder, there are known processes wherein aluminum chloride and zirconium chloride are reacted in a gaseous phase to obtain an alumina-zirconia powder [Japanese Patent Public Disclosure (Laid-Open Publication) Nos. 128268/1984 and 131860/1985]. These processes, however, have the following drawbacks. Since the starting materials are chlorides, the materials used for reactors must have corrosion resistance; chloride ion inevitably comes into the reaction product; separation of chloride ion is difficult; and for the chloride ion released out of the reaction system, measures must be taken for prevention of the public hazard potentially caused by the chloride ion. When the reaction product is calcined in order to remove the chloride ion, agglomeration of the product occurs because the product is a fine powder. This requires adequate disintegration, particularly when the product is used as, for example, a structural material. Thus, when an alumina-zirconia composite powder is mass-produced in a reaction system wherein chlorine is generated from the chloride ion as a by-product, there are many problems to be solved, which inevitably increases costs.
Meanwhile, when an alumina powder and a zirconia powder are merely mixed and the resulting mixture is molded and heated to obtain a sintered material, uniform dispersion of zirconia in alumina does not take place, thus making sintering difficult. Hence, elevation of the sintering temperature becomes necessary. As a result, growth of alumina grains in the sintered material is promoted and the sintered material suffers from reduced mechanical strength. Consequently, the sintered material is not suitable for use as, for example, a structural material.
There is also a method for obtaining an alumina-zirconia sintered material, wherein alumina and zirconia each purified and having a high purity are separately ground and sifted and then are mixed with each other and sintered. The sintered material thus obtained, however, has poor sinterability. Therefore, in order to obtain a sintered material of high density, there is usually employed hot pressing or hot isostatic pressing wherein sintering is conducted not only under a high temperature but also under an applied pressure.